Measuring apparatus

ABSTRACT

A measuring apparatus includes a heating unit for applying heat to a first point within a workpiece or on a surface of a workpiece and propagating the heat to a second point within the workpiece or on the surface of the workpiece. The measuring apparatus further includes a measuring unit for measuring a displacement of the surface of the workpiece at the second point to which the heat has been propagated, and an analyzing unit for analyzing a structure of the workpiece based on the displacement measured by the measuring unit in consideration of a distance between the first point and the second point.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a measuring apparatus, and moreparticularly to a measuring apparatus for measuring the thickness of athin film formed on a surface of a workpiece such as a semiconductorsubstrate.

2. Description of the Related Art

In recent years, a higher integration of a semiconductor device hasrequired the narrower wiring and the multilayer wiring, and hence it isnecessary to make a surface of a semiconductor substrate highlyplanarized. Specifically, finer interconnections in highly integratedsemiconductor devices have led to the use of light with shorterwavelengths in photolithography, so that a tolerable difference ofelevation at the focal point on the substrate becomes smaller in thelight with shorter wavelengths. Therefore, a difference of elevation atthe focal point should be as small as possible, i.e., the surface of thesemiconductor substrate is required to be highly planarized. Onecustomary way of planarizing the surface of the semiconductor substrateis to remove irregularities (concaves and convexes) on the surface ofthe semiconductor substrate by a chemical mechanical polishing (CMP)process.

In the chemical mechanical polishing process, after a surface of asemiconductor substrate has been polished for a certain period of time,the polishing process should be finished at a desired position ortiming. For example, some integrated circuit designs require aninsulating film (layer) of SiO2 or the like to be left on a metallicinterconnection of copper, aluminum, or the like. Since a metallic layeror other layers are further deposited on the insulating layer in thesubsequent process, such an insulating layer is referred to as aninterlayer. In this case, if the semiconductor substrate is excessivelypolished, the lower metallic layer is exposed on the polished surface.Therefore, the polishing process needs to be finished in such a statethat a predetermined thickness of the interlayer remains unpolished.

According to another polishing process, interconnection grooves having acertain pattern are formed in a surface of a semiconductor substrate,and a cooper (Cu) layer is deposited on the semiconductor substrate tofill the interconnection grooves filled with copper or copper alloy, andthen unnecessary portions of the Cu layer are removed by a chemicalmechanical polishing (CMP) process. Specifically, the Cu layer on thesemiconductor substrate is selectively removed by the chemicalmechanical polishing process, leaving only the Cu layer in theinterconnection grooves. More Specifically, the Cu layer is required tobe removed until an insulating layer of SiO₂ or the like is exposed insurfaces other than the interconnection grooves.

In such cases, if the semiconductor substrate is excessively polisheduntil the Cu layer in the interconnection grooves is removed togetherwith the insulating layer, then the resistance of the circuits on thesemiconductor substrate would be so increased that the semiconductorsubstrate might possibly need to be discarded, resulting in a large lossof resources. Conversely, if the semiconductor substrate isinsufficiently polished to leave the copper layer on the insulatinglayer, then interconnections on the semiconductor substrate would not beseparated from each other as desired, but a short circuit would becaused between those interconnections. As a result, the semiconductorsubstrate would be required to be polished again, and hence itsmanufacturing cost would be increased. The above problems also occurwhen another metallic film of aluminum or the like is formed on asemiconductor substrate and polished by the CMP process.

Therefore, it has heretofore been proposed to detect an end point of theCMP process with use of a measuring apparatus having an electric currentmeter, an eddy current sensor, an optical sensor, or the like formeasuring the thickness of an insulating film or a metal film formed ona polished surface to detect when the CMP process is to be finished. Ina deposition process such as a plating process or a chemical vapordeposition (CVD) process, it has also been proposed to measure thethickness of a thin film deposited on a substrate to detect an end pointof the process, as with the CMP process.

As semiconductor devices have been more highly integrated, the measuringapparatus has been required to measure the film thickness with higheraccuracy. The need for such a highly accurate measuring apparatus hasbeen increased not only in the field of semiconductor fabrication, butalso in other industrial fields.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above drawbacks. Itis therefore an object of the present invention to provide a measuringapparatus which can measure the thickness of a film formed on aworkpiece such as a semiconductor substrate or the like with highaccuracy.

According to a first aspect of the present invention, there is provideda measuring apparatus comprising: a heating unit for applying heat to afirst point within a workpiece or on a surface of a workpiece andpropagating the heat to a second point within the workpiece or on thesurface of the workpiece; a measuring unit for measuring a displacementof the surface of the workpiece at the second point to which the heathas been propagated; and an analyzing unit for analyzing a structure ofthe workpiece based on the displacement measured by the measuring unitin consideration of a distance between the first point and the secondpoint.

According to a second aspect of the present invention, there is provideda measuring apparatus comprising: a strain applying unit for applying astrain to a first point within a workpiece or on a surface of aworkpiece and propagating the strain to a second point within theworkpiece or on the surface of the workpiece; a measuring unit formeasuring a displacement of the surface of the workpiece at the secondpoint to which the strain has been propagated; and an analyzing unit foranalyzing a structure of the workpiece based on the displacementmeasured by the measuring unit in consideration of a distance betweenthe first point and the second point.

According to a third aspect of the present invention, there is provideda polishing apparatus comprising: a polishing table having a polishingsurface; a top ring for holding and pressing a workpiece to be polishedagainst the polishing surface; and the above measuring apparatus formeasuring the thickness of a film formed on a surface of the workpiece.

With the above arrangement, the structure of a workpiece can be measuredby a novel process which has not heretofore been available.Particularly, the measuring apparatus according to the present inventioncan measure the thickness of a metal film of W, Al, Ta, Cu, Ti, or thelike, a nitride film of TaN, TiN, SiN, or the like, an oxide film ofSiO₂ or the like, a film of polycrystalline silicon, a BPSG film, or aplasma TEOS oxide film formed on a semiconductor substrate. Themeasuring apparatus according to the present invention can also detectan end point of any process in various CMP apparatus for polishingsubstrates having shallow trenches (STI), interlayer insulating films(ILD, IMD), Cu films, W films, or the like, and various platingapparatus and CVD apparatus for depositing such films on the substrates.

The strain applying unit may utilize a sound wave, an ultrasonic wave,or an electromagnetic wave to apply the strain to the first point.

The above and other objects, features, and advantages of the presentinvention will become apparent from the following description when takenin conjunction with the accompanying drawings which illustrate preferredembodiments of the present invention by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual view explanatory of the principles of a measuringapparatus according to the present invention;

FIG. 2 is a conceptual view explanatory of the principles of a measuringapparatus according to an embodiment of the present invention;

FIG. 3 is a vertical cross-sectional view showing a polishing apparatusincorporating a measuring apparatus according to the present invention;

FIG. 4 is a vertical cross-sectional view showing an arrangement of themeasuring apparatus shown in FIG. 3;

FIG. 5 is a vertical cross-sectional view showing a polishing apparatusincorporating a measuring apparatus according to another embodiment ofthe present invention;

FIG. 6 is a vertical cross-sectional view showing a polishing apparatusincorporating a measuring apparatus according to still anotherembodiment of the present invention; and

FIG. 7 is a vertical cross-sectional view showing a polishing apparatusincorporating a measuring apparatus according to still anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, the principles of a measuring apparatus according to the presentinvention will be described below with reference to FIG. 1.

As shown in FIG. 1, when a strain applying unit 1 applies a strain to apoint A within a workpiece S or on a surface of the workpiece(substance) S, the strain applied to the point A is propagated aroundthe point A. The strain is propagated in a manner specific to theproperties (structure) of the workpiece S. For example, the propagationspeed or magnitude of the strain at a portion of the workpiece S thathas a thickness different from the thickness of another portion thereofis different from the propagation speed or magnitude of the strain atthe other portion. Accordingly, in the case where the strain ispropagated from the point A to a point B spaced from the point A, thestrain propagated to the point B depends on the structure of theworkpiece S from the point A to the point B. If the strain propagated tothe point B, i.e., the displacement at the point B, is measured andanalyzed with a measuring unit 2, then the structure of the workpiece Sfrom the point A to the point B can be specified based on the analyzedresults. The measuring apparatus according to the present invention isdesigned to determine the structure of a workpiece with use of thepropagation of a strain through the workpiece as described above.

Heat may be used to apply a strain to a point within a workpiece or on asurface of the workpiece. For example, a laser beam L₁ is applied to apoint A on a workpiece S to locally heat the point A, as shown in FIG.2. When the point A is heated, the point A is thermally expanded andhence a strain is produced at the point A. This thermal expansion is notin equilibrium and hence causes a thermal diffusion which transfers heataround the point A. As the heat is thus propagated around the point A inthe workpiece S, a strain (displacement) is also propagated around thepoint A in the workpiece S. The propagation speed and magnitude of thestain differ depending on the properties (structure) of the workpiece S.Therefore, the structure of the workpiece S from the point A to a pointB spaced therefrom can be specified by measuring and analyzing adisplacement propagated to the point B with a laser beam L₂ emitted froman optical displacement gauge. If there is a scratch or foreign matterwithin the workpiece S or on a surface of the workpiece S, then the heatis reflected, refracted, or attenuated by the scratch or foreign matter,thereby causing unusual propagation of heat. Therefore, by analyzing adisplacement caused by the propagation of heat, it is also possible tospecify the position, size, and structure of a scratch, a defect, aninterconnection defect, or foreign matter within the workpiece or on thesurface of the workpiece. While the principles of the measuringapparatus according to the present invention have been described abovewith respect to the thermal expansion, the present invention is alsoapplicable to a volumetric change (strain) of a workpiece due to heat,such as thermal shrinkage.

A measuring apparatus according to embodiments of the present inventionbased on the above principles will be described below. In the followingembodiments, the measuring apparatus utilizes heat to apply a strainwithin a workpiece or on a surface of the workpiece. However, a strainmay be applied with use of various other means. For example, a soundwave, an ultrasonic wave, or an electromagnetic wave may be used toapply a strain within a workpiece or on a surface of the workpiece. Inthe following embodiments, the measuring apparatus according to thepresent invention is incorporated in a polishing apparatus for polishinga surface of a semiconductor substrate. However, the present inventionis not limited to use in the polishing apparatus.

FIG. 3 is a vertical cross-sectional view showing a whole arrangement ofa polishing apparatus incorporating a measuring apparatus according tothe present invention. As shown in FIG. 3, the polishing apparatuscomprises a polishing table 20 having a polishing pad 10 attached to anupper surface thereof, and a top ring 30 for holding and pressing asemiconductor substrate W as a workpiece to be polished, against anupper surface of the polishing pad 10. The upper surface of thepolishing pad 10 serves as a polishing surface held in sliding contactwith the semiconductor wafer W as a workpiece to be polished. Thepolishing surface may be constituted by an upper surface of a fixedabrasive plate comprising abrasive fine particles of CeO₂ or the likewhich are fixed by a binder of resin or the like.

The polishing table 20 is coupled to a motor 21 disposed therebelow, andcan be rotated by the motor 21 about its own axis as indicated by thearrow. The polishing apparatus also has a polishing liquid supply nozzle22 disposed above the polishing table 20 for supplying a polishingliquid Q onto the polishing pad 10.

The top ring 30 is coupled to the lower end of a top ring shaft 31 whichis connected to a motor and a lifting/lowering cylinder (not shown). Thetop ring 30 is vertically movable by the lifting/lowering cylinder androtatable about its own axis by the motor, as indicated by the arrows.The top ring 30 has, on the lower surface thereof, an elastic pad 32formed of polyurethane or the like. The semiconductor substrate W to bepolished is attracted to and held on the lower surface of the elasticpad 32 under vacuum. While rotating about its own axis, the top ring 30presses the semiconductor substrate W held on the lower surface of theelastic pad 32 against the polishing pad 10 under a given pressure. Thetop ring 30 has a cylindrical guide ring 33 at an outer circumferentialedge of a lower portion thereof for preventing the semiconductorsubstrate W from being dislodged from the lower surface of the top ring30 during the polishing process.

The polishing pad 10 has a light-transmittable member 11 made of amaterial having a high light transmittance which is fitted into a holedefined in the polishing pad 10, and a measuring apparatus 40 disposedbelow the light-transmittable member 11 for measuring the thickness ofan insulating film or a metal film formed on the polished surface of thesemiconductor substrate W to detect when the polishing process is to befinished (i.e., the end point of the polishing process).

FIG. 4 is a schematic view showing the arrangement of the measuringapparatus 40. As shown in FIG. 4, the measuring apparatus 40 comprises aheating unit 50 for applying a laser beam (heating light) L₁ to thepolished surface (thin film F) on the semiconductor substrate W to heatthe polished surface, a measuring unit 60 for measuring a displacementof the polished surface of the semiconductor substrate W in the verticaldirection (the direction of thickness), and a controlling/analyzing unit70 for controlling the heating unit 50 and analyzing data measured bythe measuring unit 60.

The heating unit 50 comprises a laser beam source 52 having a laserdiode therein for applying the laser beam (heating light) L₁ to thepolished surface of the semiconductor substrate W held by the top ring30, and a laser beam source driver 51 for supplying a drive current tothe laser diode of the laser beam source 52 to cause laser oscillation.The laser beam L₁ is applied to a point A on the thin film F formed onthe semiconductor substrate W to heat and thermally expand the point A.While a single point of the thin film F may be heated by the laser beamL₁, it is preferable to heat a plurality of points on the thin film F.The laser beam L₁ for heating may continuously be applied to the thinfilm F, or may intermittently be applied to the thin film F (i.e., thelaser beam L₁ may comprise a pulsed or modulated laser beam).

The laser beam L₁ has a wavelength determined based on the absorptioncoefficient of the semiconductor substrate W. The wavelength of thelaser beam L₁ should preferably be in the range from 630 nm to 1.55 μm,and more preferably in the range from 800 nm to 810 nm. The spotdiameter of the laser beam L₁ on the point A should preferably be about30 μm. An LED having an increased output capability may be used as alight source for applying a light beam having a wider wavelength range.

The measuring unit 60 comprises an optical heterodyne displacement gaugefor measuring a minute displacement with a laser beam (measuring light)L₂. Specifically, the measuring unit 60 has a probe 61 and a maindisplacement gauge 62. The probe 61 applies the laser beam L₂ to ameasuring point B on the semiconductor substrate W, and receives a laserbeam L₃ reflected from the measuring point B. A displacement of the thinfilm F on the semiconductor substrate W is measured based on a phasedifference between the applied laser beam L₂ and the reflected laserbeam L₃. In the present embodiment, the optical displacement gauge usedas the measuring unit 60 has a measuring resolution of 1 nm and asampling interval which can be selected in the range from 10microseconds to 1 millisecond.

The controlling/analyzing unit 70 has a function to control theoscillation of the laser beam L₁ in the laser beam source driver 51 ofthe heating unit 50, and a function to perform a given processingprocess on a measured signal from the measuring unit 60. Thecontrolling/analyzing unit 70 analyzes the thickness of the thin film Fat the point B based on the measured signal from the measuring unit 60,i.e., the displacement of the thin film F at the point B, inconsideration of the distance between the points A and B and theproperties of the semiconductor substrate W, and detects when thepolishing process is to be finished (i.e., the end point of thepolishing process) based on the analyzed thickness of the thin film F.

The laser beam L₂ emitted from the measuring unit 60 may befrequency-modulated. When the frequency-modulated laser beam L₂ isapplied to the semiconductor substrate W, a frequency-modulated laserbeam L₃ is reflected from the thin film F. A high S/N ratio (signalselectivity) can be achieved by extracting an amplitude and a positionin synchronism with the modulation frequency from a signal which isproduced from the reflected beam L₃. Further, information of thesemiconductor substrate W along its depth can be obtained by varying themodulation frequency.

According to the present invention, the structure of a workpiece can bemeasured by a novel process which has not heretofore been available.Particularly, the measuring apparatus according to the present inventioncan measure the thickness of a metal film of W, Al, Ta, Cu, Ti, or thelike, a nitride film of TaN, TiN, SiN, or the like, an oxide film ofSiO₂ or the like, a film of polycrystalline silicon, a BPSG film, or aplasma TEOS oxide film formed on a semiconductor substrate. Themeasuring apparatus according to the present invention can also detectan end point of any process in various CMP apparatus for polishingsubstrates having shallow trenches (STI), interlayer insulating films(ILD, IMD), Cu films, W films, or the like, and various platingapparatus and CVD apparatus for depositing such films on the substrates.

When the measuring apparatus according to the present invention isincorporated in the polishing apparatus, the polished substrate can bemeasured for film thickness, defects, erosion, and dishing. The measureddata can be used to closed-loop control the polishing time forstabilizing the polishing process. The measuring apparatus according tothe present invention can measure the film thickness (absolute filmthickness) on the semiconductor substrate and changes in the filmthickness (relative film thickness) on the semiconductor substrate.Therefore, the accurate (absolute) film thickness on the semiconductorsubstrate can be measured during a semiconductor fabrication processsuch as a polishing process. Thus, the semiconductor substrate can bepolished while the film thickness is being measured to detect the endpoint of the polishing process (in-situ measuring process), and hencethe total number of processing steps can be made smaller than aconventional process in which the film thickness is measured in such astate that the polishing process is temporarily stopped (ex-situmeasuring process).

In the above embodiment, the thickness of a thin film formed on asemiconductor substrate is measured. However, the present invention isnot limited to semiconductor substrates, but is also applicable to anyworkpieces. The measuring apparatus according to the present inventionis not limited to the measurement of the thickness of a film, but can beused for a general structural analysis of workpieces.

In the above embodiment, the measuring apparatus 40 is disposed belowthe polishing pad 10. However, the arrangement of the measuringapparatus is not limited thereto. FIG. 5 shows another example where ameasuring apparatus 100 is embedded in the top ring 30. The measuringapparatus 100 applies a heating light and a measuring light to thereverse side of a semiconductor substrate W. FIG. 6 shows still anotherexample where a measuring apparatus 110 is disposed outside of thepolishing table 20. For measuring a film thickness, a semiconductorsubstrate W held by the top ring 30 is positioned so as to partiallyproject outwardly from the outer circumferential edge of the polishingtable 20. The measuring apparatus 110 applies a heating light and ameasuring light to the exposed portion of the polished surface of thesemiconductor substrate W in such a state that the polished surface ofthe semiconductor substrate W is thus partially exposed out of the outercircumferential edge of the polishing table 20.

FIG. 7 is a schematic view showing a polishing apparatus incorporating ameasuring apparatus according to another embodiment of the presentinvention. In the polishing apparatus shown in FIG. 7, a semiconductorsubstrate W is held on a substrate holder 120 in such a state that thepolished surface of the semiconductor substrate W faces upwardly. Aholder 122 is positioned above the semiconductor substrate W, and apolishing pad 124 is attached to the lower surface of the holder 122.The lower surface of the polishing pad 124 is held in sliding contactwith the upper surface of the semiconductor substrate W to polish theupper surface of the semiconductor substrate W. Specifically, the holder122 presses the polishing pad 124 against the upper surface of thesemiconductor substrate W, and is simultaneously rotated about its shaft126 and is also translated or revolved in a plane perpendicular to theshaft 126, for thereby polishing the entire surface of the semiconductorsubstrate W. A measuring apparatus 128 according to the presentinvention is disposed above the semiconductor substrate W, and applies aheating light and a measuring light to the upper polished surface of thesemiconductor substrate W.

The measuring apparatus according to the present invention may beincorporated in a buffing apparatus for buffing a substrate. In such acase, the measuring apparatus introduces a heating light and a measuringlight into the buffing apparatus through an optical fiber to apply theheating light and the measuring light to the substrate which is beingbuffed.

In each of the above measuring apparatuses, the heating light and/or themeasuring light may be applied to the substrate while the heating lightand/or the measuring light is being moved as needed.

In the above embodiments, an optical heterodyne displacement gauge isused as a measuring unit for measuring a displacement of the surface ofa substrate. However, any of various other displacement gauges may beused as a measuring unit. While a displacement caused by the propagationof heat is measured in the above embodiments, a thermoelastic wave maybe measured with a piezoelectric element, or an acoustic wave or a lightwave such as infrared radiation may be measured. Measured results from aplurality of measured entities may be combined with each other toenhance the accuracy of the measurement.

In the above embodiments, heat is used to apply a strain to a surface ofa workpiece. However, a strain may be applied to a surface of aworkpiece with use of a sound wave, an ultrasonic wave, or anelectromagnetic wave.

When a strain is applied to a point within a workpiece, an internalstructure of the workpiece can be analyzed based on the strain appliedto the point within the workpiece. For example, in the case whereinterconnections are provided in a plurality of layers on asemiconductor substrate, an electromagnetic wave may be applied to aselected one of the layers to apply a strain to that one of the layers,for selectively analyzing the structure of that layer.

Although certain preferred embodiments of the present invention havebeen shown and described in detail, it should be understood that variouschanges and modifications may be made therein without departing from thescope of the appended claims.

What is claimed is:
 1. A measuring apparatus comprising: a heating unitfor applying a frequency-modulated laser beam to apply heat to a firstpoint within a workpiece or on a surface of a workpiece and propagatingthe heat to a second point within the workpiece or on the surface of theworkpiece; a measuring unit for measuring a displacement of the surfaceof the workpiece at said second point to which said heat has beenpropagated; and an analyzing unit for analyzing a structure of saidworkpiece based on the displacement measured by said measuring unit inconsideration of a distance between said first point and said secondpoint.
 2. A measuring apparatus according to claim 1, wherein saidmeasuring unit measure a reflected laser beam which is reflected fromthe workpiece, wherein said analyzing unit is configured to extract anamplitude in synchronism with a modulated frequency of thefrequency-modulated laser beam from the reflected laser beam.
 3. Ameasuring apparatus according to claim 1, wherein thefrequency-modulated laser beam is varied in modulation frequency,wherein said analyzing unit outputs information of the workpiece alongits depth.
 4. A measuring apparatus comprising: a strain applying unitfor applying a frequency-modulated wave to apply a strain to a firstpoint within a workpiece or on a surface of a workpiece and propagatingthe strain to a second point within the workpiece or on the surface ofthe workpiece; a measuring unit for measuring a displacement of thesurface of the workpiece at said second point to which said strain hasbeen propagated; and an analyzing unit for analyzing a structure of saidworkpiece based on the displacement measured by said measuring unit inconsideration of a distance between said first point and said secondpoint.
 5. A measuring apparatus according to claim 4, wherein saidstrain applying unit utilizes a sound wave to apply the strain to saidfirst point.
 6. A measuring apparatus according to claim 4, wherein saidstrain applying unit utilizes an ultrasonic wave to apply the strain tosaid first point.
 7. A measuring apparatus according to claim 4, whereinsaid strain applying unit utilizes an electromagnetic wave to apply thestrain to said first point.
 8. A measuring apparatus according to claim4, wherein said measuring unit measures a reflected wave which isreflected from the workpiece, wherein said analyzing unit is configuredto extract an amplitude in synchronism with a modulation frequency ofthe frequency-modulated wave from the reflected wave.
 9. A measuringapparatus according to claim 4, wherein the frequency-modulated wave isvaried in modulation frequency, wherein said analyzing unit outputsinformation of the workpiece along its depth.
 10. A polishing apparatuscomprising: a polishing table having a polishing surface; a top ring forholding and pressing a workpiece to be polished against said polishingsurface; and a measuring apparatus for measuring the thickness of a filmformed on a surface of the workpiece, said measuring apparatuscomprising: a heating unit for applying a frequency-modulated laser beamto apply heat to a first point within the workpiece or on the surface ofthe workpiece and propagating the heat to a second point within theworkpiece or on the surface of the workpiece; a measuring unit formeasuring a displacement of the surface of the workpiece at said secondpoint to which said heat has been propagated; and an analyzing unit foranalyzing a structure of said workpiece based on the displacementmeasured by said measuring unit in consideration of a distance betweensaid first point and said second point.
 11. A polishing apparatusaccording to claim 10, wherein said measuring unit measures a reflectedlaser beam which is reflected from the workpiece, wherein said analyzingunit is configured to extract an amplitude in synchronism with amodulation frequency of the frequency-modulated laser beam from thereflected laser beam.
 12. A polishing apparatus according to claim 10,wherein the frequency-modulated laser beam is varied in modulationfrequency, wherein said analyzing unit outputs information of theworkpiece along its depth.
 13. A polishing apparatus comprising: apolishing table having a polishing surface; a top ring for holding andpressing a workpiece to be polished against said polishing surface; anda measuring apparatus for measuring the thickness of a film formed on asurface of the workpiece, said measuring apparatus comprising: a strainapplying unit for applying a frequency-modulated wave to apply a strainto a first point within the workpiece or on the surface of the workpieceand propagating the strain to a second point within the workpiece or onthe surface of the workpiece; a measuring unit for measuring adisplacement of the surface of the workpiece at said second point towhich said strain has been propagated; and an analyzing unit foranalyzing a structure of said workpiece based on the displacementmeasured by said measuring unit in consideration of a distance betweensaid first point and said second point.
 14. A polishing apparatusaccording to claim 13, wherein said strain applying unit utilizes asound wave to apply the strain to said first point.
 15. A polishingapparatus according to claim 13, wherein said strain applying unitutilizes an ultrasonic wave to apply the strain to said first point. 16.A polishing apparatus according to claim 13, wherein said strainapplying unit utilizes an electromagnetic wave to apply the strain tosaid first point.
 17. A polishing apparatus according to claim 13,wherein said measuring unit measures a reflected wave which is reflectedfrom the workpiece, wherein said analyzing unit is configured to extractan amplitude in synchronism with a modulation frequency of thefrequency-modulated wave from the reflected wave.
 18. A polishingapparatus according to claim 13, wherein the frequency-modulated wave isvaried in modulation frequency, wherein said analyzing unit outputsinformation of the workpiece along its depth.